Abstract

Randomly oriented short and low density conical carbon nanotubes (CNTs) were prepared
on Si substrates by tubular microwave plasma enhanced chemical vapor deposition process
at relatively low temperature (350–550 °C) by judiciously controlling the microwave
power and growth time in C2H2 + NH3gas composition and Fe catalyst. Both length as well as density of the CNTs increased
with increasing microwave power. CNTs consisted of regular conical compartments stacked
in such a way that their outer diameter remained constant. Majority of the nanotubes
had a sharp conical tip (5–20 nm) while its other side was either open or had a cone/pear-shaped
catalyst particle. The CNTs were highly crystalline and had many open edges on the
outer surface, particularly near the joints of the two compartments. These films showed
excellent field emission characteristics. The best emission was observed for a medium
density film with the lowest turn-on and threshold fields of 1.0 and 2.10 V/μm, respectively.
It is suggested that not only CNT tip but open edges on the body also act as active
emission sites in the randomly oriented geometry of such periodic structures.